This invention relates to a semiconductor device and a method of manufacturing the same and, in particular, to an element-isolated semiconductor device and a method of manufacturing the same.
Conventionally known is a semiconductor device using a composite substrate in which a silicon layer is positioned on a silicon substrate with disposing a silicon oxide or other dielectric layer therebetween. Composite substrates of this type are called, in general, SOI (silicon on insulator).
For fabricating semiconductor devices using a SOI substrate, the LOCOS (local oxidation of silicon) method, which is one of selective oxidation methods, or the field shield method is used as an element isolating method.
FIGS. 7 and 8 illustrate a structure of a semiconductor device in which elements are isolated by the LOCOS method. FIG. 7 is a plan view, and FIG. 8 is a cross-sectional view taken along the II-II' line of FIG. 7.
As shown in FIGS. 7 and 8, an SOI substrate is made up of a dielectric layer 602 formed on a semiconductor substrate 601 and a semiconductor layer 603 formed on the dielectric layer 602. The semiconductor layer 603 has formed n-regions (n.sup.+ -regions or n.sup.- -regions) 605, 606 behaving as source and drain regions at opposite sides of an i-region (intrinsic region) behaving as an active region to make up a transistor element region 620. The semiconductor layer 603 also has formed other n-regions (n.sup.+ -regions or n.sup.31 -regions) 608, 609 at opposite sides of an i-region 607 to make up another transistor element region 621. The remainder region of the semiconductor layer 603 other than the element regions 620, 621 is a field oxidation film 610 made by the LOCOS method for isolating the elements.
Deposited on the semiconductor layer 603 is an insulation film 611. Formed above the i-regions 604, 607 are gate electrodes 612, 613 via the insulation film 611.
As shown in FIG. 8, an inter-layer insulation film 614 overlies the insulation film 611 and the gate electrodes 612, 613. Contact holes extend through the inter-layer insulation film 614 and the insulation film 611 to expose the n-regions 605, 606, 608 and 609. A conductive material is deposited in the contact holes to form electrodes 615, 616, and so on.
This structure having the field oxidation film 610 as the element isolation film can prevent conduction between the element region 620 and the element region 621 through n-type conduction channels (namely, conduction between the n-regions 605, 606 and the n-regions 608, 609) and can isolate the elements from each other.
In the conventional semiconductor device shown in FIGS. 7 and 8, the field oxidation film 610 is made by oxidizing the semiconductor layer 603 by the LOCOS method. It results in producing outer marginal portions around the element regions 620 and 621, which gradually become thinner toward the extremities, as shown at 620a and 621a in FIGS. 8 and 9. That is, as the outer marginal region 620a or 621a becomes thinner (as a portion of the outer marginal region 620a or 621a is nearer to its extremity), it becomes more distant from the gate electrode 612 or 613 and less controlled by them.
There also occurs a phenomenon called segregation in which the field oxidation film absorbs boron atoms from the p-regions 620a and 621a in the outer marginal portions during oxidation. As a result, the outer marginal portions become liable to invert due to a decrease in impurity concentration, and these portions result in behaving as parasitic transistors.
Therefore, in the conventional semiconductor device shown in FIGS. 7 and 8, the threshold value drops in the outer marginal portions 620a, 621a, which permits the parasitic transistors to turn on before the true transistors are turned on, and causes a leak current to flow, which makes ON/OFF control of the transistors by the gate electrodes 612, 613 difficult. The leak current appears as a leak current in the element regions 62O, 613, namely, as the leak current i.sub.1 between the n-region 605 and the n-region 606 and the leak current i.sub.2 between the n-region 608 and the n-region 609.
The leak currents i.sub.1 and i.sub.2 may occur in any semiconductor device having a film that becomes thinner in outer marginal portions of element regions also when another method is used for isolation of elements in lieu of the LOCOS method.
The leak currents i.sub.1, i.sub.2 could be prevented by using the field shield method in lieu of the LOCOS method for element isolation. It, however, complicates the manufacturing process and increases the manufacturing cost of semiconductor devices.